Novartis said it does not know of any confirmed PML cases in patients treated with Gilenya, also known as fingolimod, who were not previously been treated with Tysabri. The company said details on the case were being submitted to the health authorities.

Elan spokeswoman Niamh Lyons said, "At this stage, we can't comment on the role that either drug might have had in this PML case." Biogen was not immediately available to comment.

"The development has to be taken seriously, but the question is more whether this is a trend, with a second or even third case coming up in the next few weeks," according to analyst Andrew Weiss, with Vontobel in Zurich.

Gilenya, which is currently the only oral MS treatment on the market, so far has been approved in more than 55 countries, with more than 25,000 patients having been prescribed it.

Industry analysts have said it could generate sales of at least $1 billion a year, helping to offset lost revenue caused by the expiry of Novartis' top-selling heart drug, Diovan.